Dust removal using alpha particle generator

Discussion in 'Digital Darkroom' started by WJT, Nov 8, 2021.

  1. WJT

    WJT Moderator

    When I switched from film to digital format I was hoping to finally be free from problems of dust contamination but, alas, that did not happen. Now, instead of dust on my film I have dust on my sensor. Usually this is not too bad but in some cases it really is troublesome. I have used various methods to remove the sensor dust, such an air bulb blower and wet cleaning, but some always remains, possibly because of static charges between the dust particles and the sensor.

    Question: Would it be possible to use something to dissipate the static, such as the Nuclespot polonium ionizer or similar, without hurting the sensor (my camera uses a CMOS sensor). I propose that one could remove the lens with the camera facing down and place it over the alpha generator for a time. My hopes being that the ionizer would knock the dust off the sensor. Any thoughts?
     
  2. Pretty doubtful, in my opinion.

    I don't know the exact mechanism of dust attached to a sensor but once it is attached it is nearly impossible to remove the tiniest particles solely with a blower. Larger particles - no problem. But the very, very tiny ones, not much success.

    There is a principle in so-called fluid dynamics that air flowing through a tube, for example, has a speed gradient where it slows down near the walls of a tube. And right at the walls, themselves, there is a very narrow zone where the air barely moves. So if the dust particles are very tiny they can sit in this protected zone.

    I know this sounds like baloney, but anyone who has used the high pressure spray at a car wash has probably seen how this works. You have some very fine road grime down near the bottom that simply will not be blasted off by the high-pressure stream. It's not stuck tightly - you could even use your fingertip to write your name in it. But just a quick wipe with a wet cloth or brush takes it right off. So I see this as similar to the situation of fine dust on a sensor.

    I think you're gonna have to physically dislodge the very-tiny particles.

    I'm from an outfit that had a large number of company-owned cameras. In our internal camera repair shop we had a full-time tech, sometimes two, cleaning sensors. We pretty much found wet cleaning the best way to go. It does take a certain technique to get it just right.

    There's another method, they call a "stamp tool," that has a sort of tacky material on the end of a handle. You press the stamp end gently against the sensor, and its tacky nature is strong enough to take all the particles away from the sensor. Then the stamp is pressed against a cleaning pad which is even tackier, and is able to clean the stamp tool for further use. It worked well on dust. But our techs found that a certain amount of oily particles were present on some sensors. So even after the stamp tool, a test photo might show some oily debris on some sensors. Which then had to be wet cleaned anyway. So they decided it was better to just start out with the full wet cleaning and be done with it.
     
    Ed_Ingold likes this.
  3. First of all, polonium ionizers were outlawed decades ago, along with self-illuminating signs. Alpha sources cannot be encapsulated and thus rendered safe.

    What I use and recommend for removing dust from a sensor is...
    1. A simple blower, strong enough to remove lint, with a filter on the intake port (eg, Rocket Blower)
    2. If that doesn't work, use an ultra fine fiber brush, which removes dust by static electricity after being charged with canned air or spinning (preferred)
    3. If that doesn't work, use PEC pads or Sensor Swabs and Eclipse fluid (refined methanol), followed by brushing as in stage 2.
    I have only reached stage 3 once or twice in the last decade, and stage 2 perhaps twice a year.

    You can test for dust too small (< 2 microns) to see with your eye by pointing the camera to the sky and pixel-peeping the results. The aperture, shutter speed and focus don't matter. Dust on the sensor will cast a shadow regardless. Warning! You will always see dust with this method. You have to decide whether it's acceptable or not.

    Air moving across a surface generates static electricity, which can make particles stick all the more. Most "dust" is actually lint from clothing, which is easily removed. Blow on a sensor with your breath, and you have made a new career opportunity cleaning that sensor. Same if you touch the micro fiber brush with your fingers.
     
  4. WJT

    WJT Moderator

    Thanks for the replies. Just a heads-up, the Nuclespot polonium ionizer is available under limited NRC license. One does not really purchase the Nuclespot but leases it under license. Regards.
     
  5. I used to use a Staticmaster brush to remove dust from negatives. It uses a Polonium 210 source and is still available at, Anti-Static Brush with Ionizing Cartridge - 1". Since Polonium 210 has a half-life of 138 days, the brush becomes progressively less effective with time and the manufacturer recommends replacement after one year. I still have my Staticmaster brush, but after fifteen to twenty years it works no better than a simple lens cleaning brush for removing dust. I have never tried using it for sensor cleaning.

    Bill C states that, "There is a principle in so-called fluid dynamics that air flowing through a tube, for example, has a speed gradient where it slows down near the walls of a tube. And right at the walls, themselves, there is a very narrow zone where the air barely moves. So if the dust particles are very tiny they can sit in this protected zone."

    The fluid dynamic region is known as a "boundary layer", the region in which fluid velocity along a surface progressively changes from zero velocity at the surface to the free stream velocity outside the boundary layer. (the boundary layer principle was first described by Ludwig Prandtl in the 1890's).
     
  6. There is a good description of how a Polonium 210 alpha source works to eliminate dust at the Oak Ridge Associated Universities (ORAU) Museum of Radiation and Radioactivity at, Static Eliminators (1960s and 1980s) | Museum of Radiation and Radioactivity

    From the introduction,
    "Static eliminators of the type sold to the general public have been used to eliminate dust on records, film negatives, lenses, etc. In general, they employ an alpha emitting radioactive source, usually polonium-210, to ionize the air immediately in front of the device. The ions produced in the air (positively charged oxygen and nitrogen molecules, and negatively charged electrons) are then attracted to the object that has the static charge. As a result of the collection of these ions by the object, the charge is reduced or eliminated. Since the object’s surface no longer has a static charge to hold on to the dust particles, the latter are easier to brush off."
     
  7. WJT

    WJT Moderator

    Thanks for the reply. I have a pretty good idea of how the ionizers work but my main question is will the alpha particle emanations harm the CMOS sensor of my camera? The alpha particles are quite massive and energetic; the Nuclespot contains a 5mCi source, which is very strong for a device available to the general public. Is there any evidence that these could harm the sensor? My camera is the Pentax 645Z. I would remove the lens, open the lens (sensor cleaning mode) and let the Nuclespot do its work.
     
  8. Personally I'm pretty doubtful, but I really have very little knowledge in this area. But my understanding is that alpha particles have very low penetrating power, often quoted to be easily stopped by a sheet of paper, etc. And it's very unlikely that you would have a sensor without at least a cover glass over it.

    I recall an old post here where the author wondered about the effect of cosmic rays on CMOS sensors. There were a couple links included in the replies (I didn't bother reading them, though). See this link:
    Long-term radiation damage to CMOS sensors?

    But back to my first post, I doubt it's gonna be very helpful unless you have somehow put a strong static charge on the sensor cover plate. And I really dunno how you would do that.

    My experience with such ionizers has been primarily with film. Where I worked we used to use a couple dozen such devices under yearly licenses from the US NRC. They were very effective at knocking down static charges. Something I would occasionally do to demonstrate (to the doubters) was to wipe a short strip of film, with a cloth, then pass it over an ashtray (yep, back in the days when cigarette smoking was very common in offices). A lot of ash would leap up and stick to the film. Then I could move one of the smaller ionizing devices to within about 6 inches of the film. Within one or two seconds the bulk of the ash would just drop off. But... a large amount of tiny dust particles would stay attached. Now, the "charge" situation with film is very different in the makeup of the gelatin, so likely doesn't correlate to how a glass cover plate acts, but the tiniest particles just didn't wanna let go. But that's a different story. I'm just making the point that the bulk charges are neutralized in just a couple of seconds, so any exposure beyond that is kind of pointless. And given the protective cover glass, along with a likely blur filter package, I'm pretty skeptical that sensor damage would be done.

    Fwiw, if you're in a room with say, 50% relative humidity, any static charge is gonna likely bleed off in 2 or 3 minutes, to the same extent that the air ionizer would do.

    Just my guesses and opinions.

    Maybe rodeo_joe has some insight; I have the impression that he had worked in a fab plant at one time.
     
  9. According to the web site that I quoted in my last post, "The Po-210 activity (in a Staticmaster brush) is often on the order of 200 to 500 uCi at the time of manufacture", so at most the brush would have 1/10 the alpha radiation of the Nuclespot. The 1 inch width of the brush, while ideal for cleaning 35mm negatives, might limit its applicability for sensor cleaning, and I do not find a narrower Staticmaster brush.

    The web site also states that, "....They (the NRC) also calculated an effective dose equivalent of 0.2 mrem per year to an individual who carried a 500 uCi static eliminator in a pocket for 2000 hours per year." The most limiting dose for radiation workers that I can locate is for pregnant women where the embryo or fetus is limited to a maximum exposure of 0.05 rem (50 mrem) per month. So, if either the Staticmaster brush or the Nuclespot source would be safe for a pregnant woman to use, does this imply that they should be OK for a cleaning a camera sensor? Although this seems likely to be the case, I do not know. There are many studies on radiation damage to CMOS sensors that one can find from an internet search, but it would take a lot of research to sort through them for a definitive answer to your question. The radiation problem of most concern for CMOS sensors is gamma radiation which is very much more penetrating than alpha particles. I think that Bill C is probably correct in stating that the cover glass over the sensor would probably stop the alpha particles.
     
    Last edited: Nov 10, 2021
  10. We have heard an excellent discourse on how alpha radiation dissipates the static charge which holds particles to a surface. I didn't say it doesn't work, rather that polonium sources are dangerous enough to remove from unregulated service. It would not present a danger to the sensor itself, due to their low penetrating power and several layers of glass covering the sensor. Likewise the boundary layer effect explains why, in part, fine particles resist removal by a blast of air. The larger ones surrender easily to a Rocket Blower.

    In the packaging industry (pharmaceuticals), ionization to remove static charges is produced by high-voltage plasma discharge. Polonium contamination is frowned upon by several federal agencies in that application.

    The effectiveness of micro-fiber brushes is largely due to imparting a static charge to the fibers with canned air or rapid spinning, which attacks even the finest loose particles. However you must clean the fibers and recharge them after one or two passes. Mine has an electric motor for that purpose.

    Spots which are stuck to the surface, such as dried water spray and oil (common in brand new DSLRs) are best removed with a solvent, in particular very pure methanol. Methanol dries quickly, which helps keep it from simply rearranging contamination. It is also highly polar, so it can act as a solvent for many inorganic substances (e.g., salt) as well as oil and residue from other solvents. You can search "eclipse fluid" for detailed procedures and precautions.
     
  11. - Some dust particles are attached by static attraction. Any mechanism that reduces the static charge on the sensor would be helpful yet the particles still need to be removed even when the attraction is negated.
    - There are many other options to neutralize) any static on the sensor already mentioned in other posts and many are much less expensive that the alpha source you mentioned
    - The alpha source does not address other particles etc that are on the sensor not related to static charge (some type of liquid droplet such as oil/water. So the alpha source is an incomplete solution at best
    - No the alpha source will not damage the sensor.

    My recommendation is to search for a better overall solution for sensor cleaning.
    Just my opinion of course.
     

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